Intro⌗

Recently I was browsing medium wave frequencies and DXing AM stations in other states. Late one night I noticed that there was a signal down around 520 kHz and thought I’d start there then work my way up. When I heard the signal I immediately knew that it was something different. This is when I discovered NAVTEX.

Overview⌗

NAVTEX, an acronym for navigational telex (navigational text messages) is a device used on-board the vessels to provide short range Maritime Safety Information in coastal waters automatically. It can be used in ships of all types and sizes. The area covered by NAVTEX can extend as far as 400 nautical miles from the broadcast station. A NAVTEX receiver on board prints out navigational and meteorological warnings and forecasts as well as urgent Marine Safety Information to ships. It forms a vital element of the Global Maritime Distress Safety System (GMDSS). NAVTEX uses the feature of radio telex or Narrow Band Direct Printing (NBDP) for the automatic broadcast of information.

Why use Navtex⌗

Navtex is a form of extra insurance and an aid in peace of mind. It is a very convenient way of monitoring navigational warnings, meteorological warnings, search and rescue information and other data for ships sailing within 200 to 400 nautical miles off the coast. It thus provides pertinent navigational and weather related information in real time. As Navtex receiver receives messages automatically it is quite user friendly. An officer of the watch does not have to monitor it regularly or be physically present at fixed time. There is also no requirement for retuning of the receiver. This not only saves time but also stops an officer from being distracted on the bridge. With the information received from the Navtex receiver, passage plan can be amended as required for the safety of the vessel. An officer of the watch can attend to any distress warning in the vicinity. He is also aware of the expected weather and can plan accordingly. Thus a Navtex forms an integral part of the bridge navigational equipments.

Frequencies used⌗

There are four main frequencies for NAVTEX. Three of them are mediumwave and on is high frequency.

How to decode⌗

Decoding NAVTEX is very similar to decoding other signals. A quick overview looks like this:

• Tune to signals
• Pipe audio to a local virtual audio cable
• Open a decoding program (I used YAND)
• Configure decoder to listen on the local virtual audio cable

That’s it! In some programs you may need to adjust a slider to line up the audio signal with the decoder. Other than that, the program will take care of the decoding. The nice thing about YAND is that it also includes a scheduling program that tells you what is currently broadcasting.

Below is an example of what it looks like when the text is being decoded.

Message format⌗

Once you get everything installed you’ll start to see some data coming in and how to make sense of the codes. Here is some info on how to translate the decode using an example.

The first line says:

ZCZC GA15

ZCZC identifies the end of the phasing and is the beginning of the message. It does not mean anything in relation to the message itself.

The next part is the 4 digit code will often be described in official documents as B1, B2, B3, B4, and these are described by the following:

• B1 = The transmitter identification character.
• B2 = Subject indicator characters.
• B3 = Message number.
• B4 = Message number.

GA42 has 3 components

• G - Identifies the station in New Orleans
• A - Means the message is a Navigational Warning type of message
• 15 - Message number

Subject indicator character (B2)⌗

The next part of the message says

CCGD8 BNM 0046-20

This is a Coast Guard identifier, which means that the message is coming from the Coast Guard. CCGD means Commander Coast Guard, Eigth District. 0046-20 means that this is the 46th such announcement from that USCG for the year 2020.

Next you’ll see the following message:

TX - GULF OF MX.
1. A R AND WH SEA BUOY HAS BEEN REP ADRIFT IN
APPROX PSN 28-31-42.00N 094-42-44.00W, APPROXIMATELY
40 NM OFF THE COAST OF FREEPORT, TX., ON JUN 4, 2020.
MARINERS ARE URGED TO USE EXTREME CAUTION IN THE AREA AND REPORT

ANY SIGHTINGS/FINDINGS TO THEIR NEAREST U.S.COAST GUARD UNIT.

2. CANCEL AT TIME//98233OZ JUN 20//

NNNN

This is the actual NAVTEX message that is meant for mariners off the coast of New Orleans where this is being broadcasted. It looks like there is a red and white bouy that could pose a danger for boats in the area.

The end of the message is asserted when the characters “NNNN” are received.

Each NAVTEX station has a 10 minute window where it transmits message. As part of the International Maritime Organization/International Hydrographic Organization Worldwide Navigation Warning Service (WWNWS), NAVTEX transmission schedules are coordinated across the world. The globe is split into 16 NAVAREA sections and each NAVAREA has its own station and transmission schedule.

In the US, I listen to stations in NAVAREA 4 and 12. Here are the stations and schedules for those. They are all on 518 kHz

Navarea 4 – West Atlantic⌗

Navarea 12 – Eastern Pacific⌗

My next goal is to automate this and see what information I can receive from stations as far as Miami.

For a detailed list of stations and times click here.